It is known from the prior art to provide steel sheet with a Zn—Al—Mg-based protective coating in order to thus increase the corrosion resistance of the steel sheet. Surprisingly, these protectively coated steel sheets nevertheless demonstrated a corrosion resistance that fluctuated to a relatively sharp degree.
Corrosion tests according to DIN EN ISO 9227 (NSS) carried out on these protectively coated steel sheets using an aqueous 5% NaCl solution whose pH-value was regulated with NaOH demonstrated the formation of a corrosion layer with hydrotalcite, hydrozincite, and simonkolleite as components (“XPS investigation on the surface chemistry of corrosion products on ZnMgAl-coated steel,” Duchoslav et al., AOFA 2012). In the corrosion layer, the concentration of hydrozincite Zn5(CO3)2(OH)6 was considerably greater than that of simonkolleite Zn5(OH)8Cl2.H2O. In addition, the corrosion layer contained hydrotalcite (Zn, Mg)6Al2(OH)16CO3.4H2O. Simonkolleite is also known to have an elevated corrosion resistance compared to hydrozincite.
In order to increase the concentration of simonkolleite, WO2012/091385A2 suggests adjusting the weight ratios of Al and Mg in the Zn-based protective coating so that the formation of simonkolleite is facilitated when corrosion occurs. According to the proposed embodiment, in the protective coating, the ratio of Al to (Mg+Al) should lie in a range from 0.38 to 0.48. Such composition requirements, however, disadvantageously incur a comparatively large amount of effort, particularly when protective coatings are to be applied to a sheet with the aid of a hot-dip immersion method—in other words, the reproducibility of the method can only be guaranteed with difficulty. In addition, such requirements most often lead only to a compromise between improved corrosion behavior on the one hand and undesirable changes in mechanical, chemical, and/or electrical properties on the other. This can significantly limit the usability of the sheet that has been protectively coated in this way.
In addition, JP 01127683A, JP 04165082A, and JP 2011168855A disclose coatings in steel sheets that contain Zn, Mg, and/or Al.